URL Replacer

<!--Write a PHP program URLReplacer.php that takes a text from a textarea and replaces all URLs with
the HTML syntax <a href= "…" ></a> with a special forum-style syntax [URL=…][/URL].-->

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>URL Replacer</title>
    <style type="text/css">
        #container {
            width: 350px;
        }
    </style>
</head>
<body>
<div id="container">
    <form method="post">
        <p><textarea name="text" cols="50" rows="8" placeholder="Enter your text..."
                     autofocus required></textarea></p>
        <input type="submit" value="Replace URL"/>
    </form>
    <p>
        <?php
        mb_internal_encoding('UTF-8');

        if (!isset($_POST['text']) || trim($_POST['text']) === '') {
            die('Please, enter some text in the Textfield on Form!!!');
        }

        $text = trim($_POST['text']);
        $text = preg_replace('/<a href=\\"([^\\"]*)\\">(.*)<\/a>/iU', "[URL=$1]$2[/URL]", $text);

        echo htmlspecialchars($text);
        ?>
    </p>
</div>
</body>
</html>